In the Source 2 engine, there are presets that determine which features and tools for the project should be enabled / disabled. Recently, the code name "TF" for a preset caught the community's attention - many thought of the shooter Team Fortress.
Blogger and dataminer Gabe Follower suggests that "TF" is not a new version of Team Fortress on the Source 2 engine, since "Valve already got burned with CS GO" and porting TF2 would be more difficult.
Also, in his opinion, it's not a full-fledged new installment of Team Fortress 3 - he believes that Valve may not have enough resources to fully support another online project; there would be a problem with dividing the community, and it's unclear what to do with the already established item market.
Gabe Follower shared some thoughts: he "has reason to believe" that Valve will return to the idea of Left 4 Dead with an open world (after the release of "new Half-Life"):
"...at best, we will get some single-player or co-op game that may end up not being related to the Team Fortress universe at all, since the plot in [TF 2] is officially finished. And at worst, the game will be a small tech demo for a VR headset or will be canceled."